1. Applied Field in Industry
This invention relates to the structure of a ladder-type ceramic filter, and more particularly to a ladder-type ceramic filter in which piezoelectric ceramic resonators having different characteristics are arranged.
2. Prior Art and the Problems Therewith
Various ladder-type ceramic filters are known having a high selectivity filter characteristic in which a plurality of piezoelectric ceramic resonators are connected in the form of a ladder.
Such ladder-type ceramic filters are constituted by arranging a plurality of resonators connected in series and a plurality of ones connected in parallel between the input and output terminals.
In general, for series and parallel resonators, there are employed resonators having resonant frequencies equal to each other and antiresonant frequencies equal to each other, respectively. In this instance, the respective differences .DELTA.F between the resonant and antiresonant frequencies of the resonators are fixed.
For example, for three series resonators, the resonant and antiresonant frequencies are set equal to 455 and 471 KHz, respectively, thus allowing the difference therebetween to be equal to 16 KHz. In addition, for three parallel resonators, a setting is made such that the resonant and antiresonant frequencies are equal to 439 and 455 KHz, respectively, thus allowing the difference therebetween to be equal to 16 KHz which is the same value as the difference for the three series resonators. These resonators thus set are used.
The filter characteristic a and the group delay characteristic b of the ladder-type ceramic filter constructed as described above are shown in FIG. 4. It is apparent from this figure that the filter characteristic a has a high ripple and great undulation. In recent years, a distortion factor and a group delay characteristic have become problems to be considered. The undulation of the waveform is a main cause of deterioration of such characteristics.
The characteristic of the resonators mentioned above is shown in FIG. 2 in terms of a reactance curve. As seen from this figure, the waveforms between the resonant frequency F.sub.1 and the antiresonant frequency F.sub.2 of the parallel resonator and between the resonant frequency F.sub.2 and the antiresonant frequency F.sub.3 of the series resonator are not compensated. This provides a cause of the occurrence of ripple.
As just described above, using resonators having uniform characteristics provides the merit that particular consideration is not taken for the arrangement of resonators, but is questionable to much extent in view of characteristics.